Controllable-pitch or adjustable-pitch propellers in which a push rod is eccentrically coupled to the or each of the angularly adjustable blades, are known, for example, from U.S. Pat. No. 4,897,056, wherein the positioning of the push rods is effected by hand, or from U.S. Pat. No. 4,599,043, in which an external position mechanism is provided for setting the blade pitch.
The push rods can project from the bores which are open at an end of the hub and are connected to the actuating mechanism. Usually at the beginning of the hub, these bore ends are closed to reduce corrosion and collecting of contaminants which may obstruct the sliding operation of the push rod.
In such cases, between the bore end and the end face of the push rod, an air-filled or gas-filled space is provided, whose volume varies with the change in pitch of the blades. The pressure variation in this space which is associated with the change in volume, acts upon the push rod and thus must be taken up or compensated by the actuating mechanism. This space cannot be enlarged without limit to reduce, for example, the pressure fluctuations because adjacent and between the housings for the push rods, exhaust gas passages may have to be provided in the propeller hub.
Different pressures in the space between the bore ends and the end faces of the push rods, however, cannot ensure a precise adjustment of the pitch. Rather, water can pass the seal and penetrate into the chamber between the push rod and any closure at the end of the push rod bore or otherwise pass into the space in which the end faces of the push rods are exposed. When water penetrates these spaces or chambers and, in particular, enters between the bore ends and the end faces of the push rods, this space is additionally diminished. In cold seasons, the water can freeze and the hub body can burst. When the body is transported from location at high external temperature to a cold location, for example, from a dry dock or trailer to a body of water, the large and rapid temperature fluctuations can create a vacuum in the region of the hub and promote the penetration of water into the pitch control mechanism. The system also is highly sensitive to the formation of ice in the wintertime or whenever the temperature drops sharply.